Fix Duplicate entries in db/schema.rb (Postgres)

https://github.com/rails/rails/issues/10327

When a schema.rb dump is created from a Postgres database containing multiple schemas, duplicate statements will be generated in db/schema.rb, which will cause errors when loading the schema.rb.

This patch simply adds "uniq" statements to the method chain to ensure duplicate statements are not output.

I would like to add a test for this, but am unsure how/if y'all would like to handle database-specific tests. I am unaware of how this issue might be duplicated with other RDMS's